1. Technical Field
This invention relates to hydraulic braking and steering systems, particularly those that incorporate a hydraulically boosted assist mechanism for the braking system in line with the hydraulic steering assist system.
2. Related Art
Many trucks with hydraulic braking systems incorporate hydraulic braking assist systems, rather than vacuum assist systems, particularly larger gasoline powered and diesel powered trucks. Such hydraulic braking assist systems are well known and sold commercially (e.g. the Hydro-Boost(trademark) system sold by Robert Bosch Corporation). Details concerning the construction and operation of such brake assist systems are shown, for example, in U.S. Pat. Nos. 4,620,750 and 4,967,643. Generally, these hydraulic braking assist systems are connected in series between the steering gear and hydraulic pump and use flow from the pump to generate the necessary backpressure to provide brake assist as needed. The flow from the pump is constant. Because of the series arrangement, the application of the brakes and engagement of the hydraulic braking assist system can affect the flow of hydraulic fluid to the steering gear, thereby affecting the amount of assist applied to the steering gear by the hydraulic steering gear assist system. Specifically, when a heavy braking load is applied, it causes an increase in backpressure to the pump which can exceed a threshold relief pressure (e.g., 1,500 psi) of the pump. Above this level, a bypass valve of the pump opens to divert a fraction of the outflow back to the intake of the pump, where the cycle continues until the backpressure from the brake assist device drops below the threshold valve. During this relief condition, a diminished flow of fluid is sent to the steering gear which may result in a detectable increase in steering effort by the operator of the vehicle to turn the steering wheel under extreme relief conditions.
It is an object of the present invention to modify the flow characteristics of such hydraulic brake and steering assist systems to avoid depleting the flow of fluid to the steering assist device under heavy braking conditions in order to maintain a generally constant steering effort of the steering wheel, irrespective of the backpressure to the pump exerted by the brake assist device. The invention thus aims to eliminate or greatly minimize any noticeable change in steering effort on the part of the operator under heavy braking conditions.
This invention comprises an improved hydraulic system for a vehicle comprising a hydraulic pump, a hydraulic braking assist system and a hydraulic steering gear assist system, whereby the discharge flow from the hydraulic pump is split between the steering gear assist system and the hydraulic braking assist system by a flow splitting means that is connected to both the hydraulic braking assist system and the hydraulic steering gear assist system and that is adapted to provide and control the flow of hydraulic fluid to both of these systems, thereby controlling the interdependence of these systems. This flow splitting means has a sensing means to sense the brake assist pressure of the fluid fed to the hydraulic braking assist system. As the hydraulic braking assist system is activated, the brake assist pressure at the sensing means increases. At a control pressure, preferably a pressure just below the relief pressure of the hydraulic pump, the flow splitter will shunt or bypass a portion of the flow around the hydraulic braking assist system directly to the steering gear assist system, thereby providing sufficient flow to operate both systems within normal design limits. The control pressure in the flow splitter may be fixed or variable. Systems which sense pressure and vary the flow of fluid, for example electronic pressure control valves, are well known, and could be adapted to vary the flow in the present invention.
The flow splitter means can be located in line at any convenient location between the pump and the hydraulic braking assist system, and is operative to divert a fraction of the flow of hydraulic fluid issued from the pump to the hydraulic steering gear assist system, as needed, depending upon the line pressure to the hydraulic braking assist system.
The invention further contemplates such an improved hydraulic system in which the sensing means further senses the steering assist pressure of the fluid fed to the hydraulic steering assist mechanism. At a second control pressure, preferably about the same as that associated with the brake assist system, the flow splitter operates to supply the required flow to operate the brake and steering assist mechanisms while shunting or bypassing an excess fraction of the available incoming flow from the pump around the steering and brake assist systems to another location in the system, preferably a fluid reservoir. The invention thus provides a sensing means with a double relief feature that operates to relieve both the braking and steering assist systems at preferably about the same control pressure, enabling a single flow control device to be used to control the relief of both the braking and steering assist systems. Such simplifies the overall design of the hydraulic system, requires fewer components and less space, and is less expensive to implement than a separate flow control device for each of the braking and steering assist systems.
The invention further contemplates such an improved hydraulic system in which the flow rate of fluid fed to the flow splitting means by the pump exceeds the flow rate required to operate the hydraulic brake assist system, and whereby the flow splitting means is operative to constantly bleed a portion of the incoming flow directly to the steering assist mechanism.
Preferably, the diversion of fluid is achieved by choking the flow port to the brake assist system in a manner that still provides the required flow rate of fluid to both the brake and steering assist systems. Under normal operating conditions, the brake assist system requires a lesser flow rate of hydraulic than that of the steering assist system, and thus the flow to the brake assist system can be choked to provide non-uniform flow matching the different flow requirements of each system. Supplying the steering assist system with a relatively greater flow under normal operating conditions has the advantage of minimizing any noticeable increase in steering effort under a condition of hard, sudden braking where a momentary spike in the flow requirement of the braking assist system occurs. With a constant bleed of fluid to the steering assist system, the needed fluid to operate the steering assist mechanism under such sudden braking condition is already present. The choke control to provide a constant bleed of fluid to the steering assist system has the further advantage of prepositioning the sensing means closer to the relief condition, whereby the flow splitting means can react more quickly to divert a fraction of flow to the steering assist mechanism when the brake assist pressure reaches the control pressure. Such provides for a smoother transition between the various operating conditions and further minimizes any change in steering effort detected by an operator of a vehicle during such transitions.